Control system



R. ADLER CONTROL SYSTEM gamas Dec. l?, 1957 Original Filed April 16. 1956 5. Sheets-Sheet l ROBERT ADLER INVENTOR.

\ HIS ATTORNEY.

R. ADLR CONTROL SYSTEM Dec. 17, 1957 5 Sheets-Sheet 2 riginal Filed April 16, 195s ROBERT ADLER JNVENTOR.

HIS ATTORNEY.

Y Dec., l?, i957 R. ADLER 2,8E7,025

CONTROL SYSTEM Original Filed April 16. 1956 5 Sheets-Sheet 5 L|64-(Reloys operaie) 0 Voltage frequency ROBERT ADLER INVENToR HIS ATTORNEY.

R. ADLER CONTROL SYSTEM Dec. 17, 1957 5 Sheets-Sheet 4 Original Filed April 16, 195B R m mm L Dm p A T of R E B O v y Dn HIS ATTORNEY.

2&3 7,925

R. DL-ER CONTROL SYSTEM Dec. 17, w57

5 Sheets-Sheet 5 Original Filed April 16, 1956 ROBERT ADLER INVENTOR.

HIS ATTORNEY.

CNTRL SYSTEM Robert Adler, Nortlreld, Ill., assignor to Zenith- Radio Corporation, a corporationof Illinois Original application April 16 11956'` Serialr No. Divided and this applicatiori'l Augiist 5,1957, Serial No 11 Claims. (Cl. 307-440) This invention is directed to a new'v and improved control system for controlling oper-ation of an electrical circuit. rlhe system is particularly valuable whenvapplied to control of one or more electricall circuits ina'v Wave-signal receiver such as a television: receiver, and isde'scribed in that connection; itis not,however,-restricted" to this particular use, but may be employedi'nlcontrolling' apparatus ina wide variety of applications.. The present: application is a division of the co-pending application of Robert Adler', Serial No. 578,333, tiled April 167,. 1956,l forl Control System, and assigned to the same assignee as the present application'.

There are many different types of electrical orelectrically=cont`rolled apparatus for which convenience and efciency of operation may be greatly enhanced by a`- remotey control system. For example, a-` televisionI receiver is best utilized whenV the observer is' seated ata substantialdistance from' the receiver, thus making itrelatively'qin'- convenient to change the station' or signal? channeltd which' the receiver is tuned' when' a' change in programs is desired, tov change the' amplitude of s'ound'- from the' receiver, to turn the receiver on and off; e'tc. According.-` ly, it is highly desirable to provide a system to' regulate the receiver operation without requiring: the'v observer" to leave the' normal viewing position.- Similarly, it isfre' quentl'y desirable'to provide for remote'control ofdoors, as on a garage, of heatingv apparatus; suchasl a furnace',. and' of other similar electrical or electrically-controlled devices. In many ofthese'applications, itisun'd'esirable' to'have aA direct cable connection from'the' remote' control station to the controlled device, since' a wir'e or cable link` is not particularly attractive' inap'p'e'a'rance" andr'm'ay' oftenx cause' accidents when extended' transversely of arr areaA where people must walk.`

Remote control systems in which operatingclaracterl istics of a radio or television receiver or other devicear'e varied in' response` to radio, acoustic',` otflightl si'ginalsflive been' employed inL thepast. Those' systems? whichiut'i-liief a portable miniature radio transmitter" have"l generally been unsatisfactory ini thattle control! sys't'ern may' be'E triggered tochange the operating! char 'cteris'tics ofi the? controlled' device by signalsernanat'ingf lsou'r'ce's' other' than the' control' transmitter. Radio-linked' remote; clon-f trol systems frequently create' objeot'ionabl'e'- interference' inv other wave-signal receivers; theya'lsoi tetidt' le'rela'` tively complex and expensive tomanufact re andreq'ir batteries or some other sourcel of`ele'ctif`icalpover at the" transmitter.

LightV impulse actuated systems are generallyeffectivte in operation, but frequently are relatively eirpeii'sive, parl" ti'cularly where al number of different' electrical circuits aretofbe controlled, sinceA the photoss'ensitive deviceseini ployedat th'e' receiving'sta'tion of the system" arerelatively costly; Systems of' thisl type' are also sometimes: s'ulfi'jec't5 to'y false actuation undef adverse ambient lighting condi?V tions'.-

Acoustic control systems;usingsignalsiwboth tlia'di'" ble and ultra-sonic' ranges, have benproposed many timesbut have-noti found general acceptance. This lack of acceptance is generally attributable to the fact that the amplitude of the signal received at the picleu'p station of the system variessubstantially as the distance betweenl the transmitting andpick-up' stationsl isl changed. Thist factor tends to make a control system based upon amplitude modulation of an acoustic carrier quite erratic' inA op'eration. I-n addition, systemsof this type arefquite frequently subject to false triggering from extraneous acoustic sig? nale.-

It is a principal object of this invention, therefore-,. tol provide a new and improved` remotely-actuate'd'- cont'iol systemi for any electrical ori electrically-'controlled device' which overcomesT the above-noted disadvantages'y of prior artarrangements.

I-t is amore specic object of the'invention to provide a newv andimproved remote control system inf which' tr'igi-A gering of the'y system' from extraneous and uncontrolled sources is effectively avoided.

le isa" corollary object of the invention to' provide ai new and improved remote' control system for aE wave' signal: receiver or similar device which may be easily" incorporated in that. device without substantially' modifying itsbasic construction.

ItJ is another object ofi the` invention to provide an acoustically actuated remote-control systemi-n-'wli'ch operation isf to' a'l substantialr extent' independent' of the am 'li`-y mue of the received signal and which effectively dis'c'r'ifmtl hates'- a'gainst extraneous signals in? the samefrequency range4 as the control signals.

rr is a further object of the invention fd provides new and improved' remote'control system of simple and i'reX- pensive'constlruction;

Accordingly-,the invention is directed to a controlE sy's-' tem adapted for remoteactu'at'ion by an-ultra`-sonic signal of predeterminedv minimum amplitude f and duration'l witli= in a predetermined restricted frequency range; the int/'en`r tion is utilized to control ani electrical' circuit' actuatable betweenat least; two d'ifferentoperating" conditions'. The' control= system' comprises anl input circuit, including a microphoneg. which generates an electrical input signal havingta'- frequency representative of the ultrasonic con"-` t-ro1 sigaar. A: limiter is" coupled' to the input circuit' and isr'employed to'develop an amplitude-limited signall having4 a'- predetermined frequency equal to' an integral multiple ofthev frequency ofthe' input' signal; the frequency of the amplitude#limited signal may be the same asthe input signal frequency, but preferably constitutes a rela'- tivel-y'lbw harmonicthereof. The system further includes a-segregation network, coupled to the limiter, for developing acontrol signal only in response to an amplitudelimited signal of predetermined minimum duration' and duty: cycle within a restricted frequency band including they amplitude-limited signal frequency, thereby distinguishing the desired"amplitude-limited signalfrorn extraneous' signals, b'oth within and without the frequency band, which may' appear in the output of the limiter. Additional means are coupled to this segregation'I net-` work and to the controlled` electrical circuit for utilizing. the control# signal to actuate the circuit between its dif# ferentl operating' conditions.

The-features of the invention which are believedI to' be" new are' set forthl with particularity in the appended' claims. The'inventiomtogether with further objectsv and' advantages thereof, may best be understood, however, by reference tothe following description taken' in conjunc'# tionwith the accompanying drawings, in which:

Figure 1 is a block diagram of a remote' control systemictnstructed inlacc'ordance with the' invention and also shows, in very simplified form, a television receiver controlled by the invention;

Figure 2 is a detailed schematic diagram of the receiver circuitry for a preferred embodiment of a remote control system constructed in accordance with the invention;

Figure 3 is an explanatory diagram showing certain operating characteristics for the frequency-discriminator circuit of Figure 2;

Figure 4 is an explanatory diagram illustrating operation of integrating circuits in the embodiment of Figure 2;

Figure 5 is a perspective view of a transmitter unit for a remote control system constructed in accordance with the invention; and

Figure 6 is a schematic diagram of a portion of anotherembodiment of the invention.

In Figure 1, a conventional television receiver 1.0 i's is shown in simplified block diagram form as an example ofthe type of electrical and/ or electrically-controlled apparatus which may be regulated by remote control systems constructed in accordance with the invention. Receiver10 includes an antenna 11 coupled to a receiving circuit unit 12; receiving circuits 12 may include the usual radio-frequency amplifier, station selector, first detector, and intermediate-frequency amplifier stages found in most conventional receivers. Receiving circuits 12 are coupled to a second detector 13 which, in turn, is coupled to a Video amplifier 14, to a sweep system 15, and to the usual audio circuits 16. The output of video amplifier 14 is coupled to the electron gun of a cathode-ray image image reproducer 17. The output of sweep system 15 is coupled to the deflection yoke of the picture tube, here illustrated schematically by coils 18 and 19. Audio circuits 16 are coupled through a relay 37 to the usual loudspeaker 20. The operating circuits of the receiver are provided with the necessary operating potentials from a receiver power supply 21 which is energized from an ordinary 115 volt 60 cycle source as indicated by the power input plug 22. A tuner motor 23 is mechanically connected to the station-selection section of receiving circuits 12 to permit remote control of this stageof the receiver, as explained in greater detail hereinafter.

The construction and operation of television receiver 10 is entirely conventional and may be varied as desired without having any effect upon the invention; accordingly, only an exteremely brief description of its operation is included here. A television signal isintercepted at antenna 11 and suitably ampliiied and detected in receiving circuits `12 to develop an intermediate-frequency signal which is supplied to second detector 13. The second detector develops a composite video signal which is supplied to video amplifier 14, sweep system 15, and audio circuit- 16. The video information portions of the signal are utilized to control the intensity of an electron beam developed in picture tube 17, whereas the synchronizing signals are employed to control deflection of the beam across the image screen of the picture tube under the iniiuence of the sweep signals developed'in system 15 and applied to deiiection yoke 18, 19. The audio portion of the composite video signal is suitably detected in circuit 16 and employed to energize loudspeaker 20.

It will be recognized by those skilled in the art that many changes may be made in the construction of receiver 10 without departing from conventional practice. For example, the information signal input for audio circuit 16 may be derived from the output stage of video amplifier 14 rather than from second detector 13; the same change may be made in the signal input circuit for sweep system 15. Then too, an electrostatic deflection system may be employed for image reproducer 17. None of these or the many other possible changes, however, have any substantial effect upon the structure and operation of the remote control system schematically illustrated in Figure l.

The remote control system .of Figure liomprises a transmitter 30 which develops four ultra-sonic acoustic signals of predetermined minimum amplitude and duration within four distinct restricted non-overlapping frequency ranges. The structure and operation of a preferred transmitter device are described hereinafter in detail in connection with Figure 5. Transmitter 30 constitutes the remote or portable portion of the control system; the remainder of the apparatus illustrated in Figure l is preferably located closely adjacent to receiver 10 and may be constructed as an integral part of the receiver. The stationary or receiving section of the control system comprises an input circuit 31 including a microphone 32 coupled to a suitable amplifier 33 of any desired number of stages. Amplifier 33 is in turn connected to a limiter circuit 34, the output stage of the limiter being coupled to a pair of series-connected'segregation networks 35 and 36. Segregation network 35 is connected to two relays 37 and 38; network 36 is similarly coupled to a pair of relays 39 and 40.

Relay 37 includes a switch 47 which is connected in series with one of the input leads for loudspeaker 20. Relay 38 comprises a switch 48 connected in series with one of the power leads between power-connection plug 22 and receiver power supply 21. Relays 39 and 40 include switches 49 and 50 respectively which are incorporated in the energizing circuits of tuner motor 23. The remote control system also includes a power supply 51 having an input circuit connected across the leads from power input plug 22; control power supply 51 is suitably connected to each of circuits 32-36 and to relays 37-40.

As indicated above, the control system is actuated by ultra-sonic signals radiated from transmitter 30; for example, in a given instance transmitter 30 may develop a signal having a frequency of approximately 40 kilocycles. Generally speaking, the transmitter comprises a simple acoustic resonator which is shock-excited to produce a signal; consequently, the signal is not of constant amplitude but decays exponentially as time progresses. To assist in explaining operation of the `control system, it may be assumed that the time constant of the resonator output signal amplitude is of the order of 0.3 seconds.

The ultra-sonic signal radiated from transmitter 30 impinges upon microphone 32; as a consequence, the input circuit comprising microphone 32 and amplifier 33 generates an electrical input signal having a frequency representative of the received acoustic signal. In the preferred embodiment to be described in greater detail hereinafter in conjunction with Figure 2, the frequency of the input signal generated in input circuit 31 is equal to that of the received ultra-sonic signal; however, in some applications it may be desirable to employ a heterodyning device in the input circuit, in which case the frequency of the electrical input signal is determined by but not necessarily equal to that of the received acoustic signal.

The electrical input'signal developed in circuit 31 is applied to limiter 34, which generates an amplitudelimited signal having a frequency equal to an integral multiple of the aforementioned electrical input signal frequency. In'this connection, it should be noted that the term integral multiple, as used throughout this specification and in the appended claims, refers to multiplication by an integer, including unity. Preferably, the amplitude-limited signal appearing at the output of limiter 34 represents a low odd harmonic of the electrical input signal such as the third or fifth harmonic. The amplitude-limited signal developed in limiter 34 is applied to segregation networks 35 and 36, which distinguish the desired amplitude-limited signal from extraneous signals, both withinand without the frequency band of the actuatingsignal, which may appear in the output from limiter 34. AIn the embodiment of Figure 1, four individual circuits in receiver 10 are regulated by the remote control system; consequently, it-is necessary to provide means in transmitter 30 to develop four distinctive ultra-sonic actuating signals. Segregation of functions is provided astucesoney a frequency Basis; thus, in the illustrated control sys tem; transmitter3)incluties-means for generating acoustic signals: atf four different frequencies. For example, the frequencies employed may be 3S, 39, 40 and 4l kcs. Segregation networks 35 and 36 are constructed to devel'op` c-ontrolsignals only in response to signals of predetermined` minimum duration and duty cycleY Within restrictedl frequency bands respectively including the' selected" operating frequencies. The received' signals areA distinguished from each other in networks 35 and 36 strictly on a frequency basis.

In order to avoid( triggering of the controlj system4 from extraneous acoustic, electrical, or magnetic` signals orv stray fields, it is also necessary that segregation circuits 35 and 36 be able to analyze the received signal on the basis of'duration and duty cycle. For this reason, the' twoi devices each include an integrating system for averaging the' signal received from limiter 31 over an extended period of time somewhat shorter than the time constant of: transmitter 30. The integratedA signal'v is thenV applied to` a threshold device which remains inoperative except during intervals when the integrated signal exceeds a` preselected minimum value. Consequently, a received signal"vvithin` the same frequency range as the acoustic actuating signal but of substantially shorter duration does not result in generation of a control signal in the output stages of networks 35 and 36. By the same token, an intermittent signal within the acoustic frequency range of the' system cannot trigger it into operation in most instances; depending upon the minimum operating level selected for the threshold devices in the segregation circuits. Of course, the receiver system can be falsely triggered' by a" received signal ofreiatively constant amplitude within the acoustic frequency range of the system; however, this'is not likely to occur inmost locations.

When the received signal is in the proper frequency range' and*y meets the system requirements with respect to' duration and duty cycle, relay 37, for example, may be actuated. A ratchet or step type relay is employed for relay 37' so that the audio system of receiver 10 may bev alternately energized and muted by successive actu ations of. the relay. Accordingly, when the sound output' from speaker 2t) exceeds the level of comfort or when silence is. momentarily desired, transmitter l may bei actuated to emit the necessary ultra-sonic signal to operate relay 37 and open the audio input circuit to speaker 20; upon the next actuation of relay 37, audio operation' is restored. A similar step-type or ratchet relay is employed as relay 3S, which serves to energize or tie-energize receiver 1t? completely. Of course, the signal l from transmitter' 36 employed to actuate relay 39- is at a' different frequency from that utilized in operating relay 37, the two different actuating signals being distinguished from eachl other on a frequency basis by circuits and'36.'

Operation of the tuner control relays 39 and 4i) is essentially similar to that of relays 37 and 38 except that the motor-control relays are preferably of the instan-y raucous-contact type. In the illustrated system, a reversible motor is employed; switch 49 of relay 393 is connected in the motor-energizing circuit employed for clockwise rotation, whereas switch 50 of relay 49 is connected' in the energizing circuit utilized for counterclockwise rotation. An acoustic signal having a frequency different from those used for control of relays 317 and 38 is employed to actuate relay 39, and an acoustic signal of a fourth distinctive frequency actuates relay 402 The motor-control circuits are essentially similar to those long used in the art in connection with other types of lremote control systems and consequently are not de'- scribed in detail in this specification.

Figure' 2' isa detailed schematic diagram-of a preferred` embodiment of the invention comprising a microphone 621015 the: variable-capacitance type; one terminal of thef microphone; is groundedE and' the other isi coupled to the` 75 62 back to ther positive or B+ terminal of the control'- power supply 51?. Cathode 70'v` of amplifier tube 63l is connected' to ground' through a bias resistor 71 which is bypassed by a capacitor 72. The suppressor electrode 7S of the tube is connected directly to the cathode, and the screen electrode 74 is connected to the B+ supply through a resistor 75, the screen being bypassed tol groundA through a capacitor 76.

The' output circuit for tube 63" comprises a parallel:- resonant circuit including an inductance' 77 and a capacitor 78'; the tuned circuit is connected in' series between the' anode 79"'of tube 63 and th'e'B+ supply. An'ode" 79 isalso coupled to the control1 electrode 80 of thepentode section 81A of a combined pentode-triode by' means of an RC coupling circuit comprising a series coupling capacitor 82'and a selflbiasing resistor 83 which connectsv control electrode to ground. Tube section 81A forms a part of theA second stage of the input amplifier of the system and includes a cathode 84 which is connected directly to ground, the suppressor electrode 85 in this amplifier stage being connected directly to the cathode'. The screen electrode 86 is coupled to a conventional biasing circuit comprising a resistor 87 which connects theV screen electrode t'o the B+ supply anda capacitor 88' bypassing the screen electrode to ground.

The output circuit for amplifier section 81A is a conventional RC coupling circuit which cou-ples the anode 91B of" tube section 81A to the control electrode 91 of.- ai

` triode tube section 81B. The couplingk circuit includes a load resistor 92 connecting anode 90 to B+, a capacitor 93- and a resistor 94 connected in series between anode and control electrode 91, and a coupling resistor 95 connecting the terminal of capacitor 93` opposite anode 90A to ground. Tri-ode sectionSlB comprises the" third and iinal stage of the input ampliiierof the system and includes a cathode 96 which is connectedv to ground and an anode 97 connected to B+ throughA a load resistor 981 Accordingly, the circuit as thus. far described4 corresponds directly to the input circuit 31 of Figure l.

In the embodiment of Figure 2, input circuit 31l is coupled` to a limiter circuit 34 bya coupling capacitor 14N) connected in series between anode 97 of tube section 81B. and the control electrode 101l of a limiter tube 102; the input circuit for tube 102 also includes a tuned circuit comprising an inductance 103 and`r a capacitor 1614' connected in parallel with each other between control electrode 191 and ground. In theillustrated embodiment, tube 192 is of the gatedbeam type comercially available under the type designation 6BN6. Limiter tube 102 includes a cathode 105 connected to `ground through an unbypassed biasing resistor 106. The limiter tube further includes a pair of `accelerating electrodes 107 and 16S disposed on opposite sides of control electrode itil.; the two accelerating electrodes are con.- nected to each other and are connected to the B+ supply through a resistor 109, being bypassed to groundV by a capacitor 110. Tube 102 further includes a second control electrode 111 and an anode or output electrode 112; the second control electrode is not utilized in op.-

eration of the limiter and may be connected to anode. 112 asshown or to ground.

Anode 112 of limiter tube 102` is returned to B+ through a circuit comprising two anti-resonant circuits 11S and 116 connected in series withl each. other. The terminal of resonant circuit 115 connected to anode 112- is. coupled to the electrical center of an inductance 117 through a couplingjcapacitor: 118', anda: capacitor 119 isconnec'ted in` parallel' with coil 117 to formt an -antifre'st onantl circuit tuned to' the same frequency as` circuit 115. Coil 117 is also inductively coupled to the inductance coil of tuned circuit 115. The opposite terminals of coils 117 are respectively connectedvto the two anodes 120 and' 121 of a double diode 122. The cathode 123 of tube 122 associated with anode 121 is connected back to the electrical midpoint of coil 117 through a resistor 124, and the cathode 126 associated with anode 120 is returned to the same point through a resistor 127. Cathodes 123 and 126 are bypassed to ground by capacitors 129 and 130 respectively and are returned to a source of negative operating potential C in control power supply 51 through two equal resistors 131 and 132 respectively. Tube 122 is thus incorporated in a conventional balanced frequency-discrirninator circuit frequently used as a detector for frequency-modulated signails.v In the present invention, however, the balanced frequency discriminator is used in a somewhat different manner than in conventional practice, as will be made more apparent in the operational description of the system included hereinafter.

The frequency-discrimination device comprising tube 122 forms a part of the first segregation network 35 (see Figure l); network. also includes further means for distinguishing between desired and undesired output signals from limiter 34 on the basis of duration and duty cycle of the received signal. A pair of resistors 133 and 134 are connected in series with each other and with cathode 123 of tube 122, and a similar pair of resistors 135 and 136 are connected in series with each other and with cathode 126. The common terminal of resistors 133 and 134 is bypassed to the common terminal of resistors 135 and 136 by a capacitor 137; the other terminal of resistor 134 is bypassed to ground through a capacitor 138, whereas the corresponding terminal of resistor 136 is bypassed to ground through a capacitor 139. Resistors 133-136 and capacitors 137--139, together with resistors 131 and 132, constitute a pair of integrating networks for developing7 potentials indicative of the average amplitudes of the signals appearing at the cathodes of the frequency discriminator comprising tube 122.

Network 35 further includes a threshold device or amplifier comprising a double triode 140. The two cathodes 141 and 142 of tube 140 are grounded; the control electrode 143 associated with cathode 141 is connected to the common terminal of resistor 134 and capacitor 13S, whereas the control electrode 144 associated with cathode 142 is similarly connected to the common terminal of resistor 136 and capacitor 139. The anode 145 of tube 140 associated with cathode 141 and control electrode 143 is returned to B-lthrough the operating coil 147 of the muting relay 37 (see Figure 1). Similarly, the other anode 14S of tube 140 is connected to the B+ supply through the operating coil 149 of the on-off relay 38 (Figure 1).

Tuned circuit 116 is incorporated in the second segregation network 36 which, as indicated in the description of Figure 1, is similar in construction to network 35. Network 36 comprises a second tuned circuit 150 coupled vto a double diode 151 and to resonant circuit 116 in the same manner as in discriminator 35; the two cathodes of tube 151 are connected to a dual integrating network 152 which in turn controls operation of a threshold amplifier comprising a double triode 153. One of the anodes 154 of amplier tube 153 is connected to the B-lsupply through the operating coil 155 of the clockwisemotor-control relay 39, whereas the other output electrode 156 of tube 153 is returned to B+ through the operating coil 158 of the counter-clockwise-motor-control relay 40.

The basic operation of the control system receiver station illustrated in Figure 2 is essentially similar to that described in connection with Figure 1; Figure 2, however, illustrates several features of the invention which, although not essential to the fundamental system concept, nevertheless provide for greatly enhanced eiectiveness in system operation as compared' with otherpossiblel embodiments. An acoustic signal impinging upon microphone 62 effectively varies the microphone capacitance and excites the three-stage amplifier comprising tubes 63, 81A, and 81B. The electrical signal variations provided by the microphone are rst amplified in tube 63, the tuned output circuit 77, 78 of the tube providing for substantial attenuation of most frequency components outside of the selected acoustic frequency range of the system (38 to 4l kilocycles in the assumed example). The electrical signal from amplifier tube 63 is further amplified in tubes 81A and 81B and constitutes the input signal applied to limiter tube 102. Further frequency selection is provided by the parallel-resonant circuit 103, 104 in the input circuit of the limiter. 4

Limiter 34, comprising tube 102, performs two distinct functions. It operates as a limiting amplifier, providing an output signal of constant amplitude over a wide range of input signal amplitudes. The tube selected for this limiter must have an output electrode current vs. control electrode voltage characteristic comprising two control electrode voltage ranges of substantially zero transconductance separated by a control electrode voltage range of high transconductance, a characteristic best achieved by a gated-beam tube such as the 6BN6 but also attainable in other conventional devices such as the 6BE6 or 6BU8. With a tube and circuit exhibiting this characteristic, the limiter functions Aalso as a harmonic generator and provides substantial output signals at the third and fifth harmonics of the input signal. The structure and operation of a harmonic generator of this type are described in detail in U. S. Patent No. 2,681,994 to Robert Adler, led September 27, 1949, issued June 22, 1954, and assigned to the same assignee as the present invention. Accordingly, a detailed description of operation of the limiter circuit is unnecessary here. It is suicient to indicate that the limiter develops an amplitudelimited signal having a frequency whichis an integral multiple of the input signal frequency; in the illustrated embodiment, the third harmonic of the input signal fre-l quency is utilized for reasons indicated hereinafter. Any other type of limiter may of course be substituted for the illustrated device, particularly where the discriminators of the system are constructed to operate at the fundamental frequency of the output signal from limiter 34. Moreover, it should be understood that one stage of the amplifier of circuit 31 may be constructed as a frequency multiplier, in which case circuit 34 may function only as a limiter.

The amplitude-limited signal from limiter 34 is supplied to the tuned circuits and 116 of the discriminators included in networks 35 and 36 respectively. The two discriminator input circuits are preferably connected in series as illustrated; this is possible because they are tuned to substantially different frequencies and each represents a relatively low impedance at the resonant frequency of the other. In the illustrated system, as in Figure 1, four acoustic signals of different frequency are utilized for four different control functions; the frequencies selected, may, for example, be 38, 39, 40, and 41 kc. respectively. With these operating frequencies, parallel-resonant circuit 115 maybe tuned to a frequency of 38.5 kc., the center frequency between the two lower-frequency signals, in which case reso-nant circuit 116 is tuned to 40.5 kc., the median for the two higher-frequency signals. Operation in this case is predicated upon use of the fundamental component of the output signal from limiter 34. Operation on the fundamental, however, presents difficult problems in feedback between the circuit elements, particularly the inductances, of discriminator devices 35 and 36 and the different stages of the input amplifier circuit, particularly the tuned circuit 77, 78 incorporated in the output circuit of amplifier tube 63. The possibility' of such regeneration difficulty is apparent from the fact that the relatively low frequencies involvedrna'ke. magnetic shielding difticult and expensive and the further fact that amplification in. the system must be; extremely high in order to provide for use of relatively weakr acoustic triggering` signals. Consequently, in the preferred system illustrated resonant circuit 115 is tuned to 115.5 kc., the third harmonic of the median frequency for the two lower-frequency signals. Similarly, circuit 116 is constructed to have a resonant frequency of 121.5 kc., the third harmonic of the median for the two higherfrequency trigger signals.

In accordance with the usual construction of frequency discriminators, the resonant circuit comprising. coil 117 and capacitor 119 is tuned to the same frequency (115.5 kc.) as resonant circuit 115 and theV coils. of the two circuits are disposed in mutual coupling relationship. Consequently, the discriminator comprising the twotuned circuits, tube 122, coupling capacitor 11S and resistors 124: and 127 has an operating. characteristic as illustrated by dash line 161? in Figure 3'-,.irr whichthe voltage appearing acrossy cathodes 123 and 126 is plotted: as av function of the frequency of the signal applied to tuned circuit 115 from limiter 34. Curve 1601 is representative of the magnitude of that voltage; however, it shouldA be understood that the polarity isf arbitrarily selected. As drawn, the curve represents the potential of cathode 123 with respect. to cathode 126; if cathode 123 had, instead, been chosen as potential reference, curve 160 would appear reversed. Resistors 131 and 132, of substantially equal resistance are connected in series across the two cathodes 123 and 126, their common terminal being returned to the C- reference voltage. Half theV discriminator output voltage, therefore, appears across each of these resistors. The voltage across resistor 131 is plotted' in Figure 3 as a function ofthe frequency of the signal applied to the discriminator, beingA illustrated by solid. line 161; its amplitude is approximately half that of the total discriminator output voltagel and includes values both positive and negative with respect to the C- reference voltage to which the common: terminal of resistors 131 and 132 is returned. The volta-ge across resistor 132 illustrated by dotted line 1-62, follows a characteristic essentially similar tothat of curve 161 except that the polarity with respect to the bias voltage is reversed.

The circuit parameters for they discrirninator circuits are so selected that the two peaks of each of voltage characteristic curves 160-162 are centered at 114- and 117 ltcs. respectively, these frequencies beingthe third harmonics of the. two acoustic frequencies (38 and 39 kcs.) employed to actuate this portion of the control system. In conventional use of the discriminator circuit as a detector for frequency-modulated signals, only the relatively linear portion of characteristic 160 centered about the median or resonant frequency of 115.5 kcs. would be employed. In the present instance, however, the. effective operating range for thefrequency discriminator is restricted to two narrow por-tions, each including one of the two peaks at 114 and 11-7 kcs. to enable the system to distinguish between these two frequencies and to discriminate against other frequencies outside the twol operating ranges. For this reason,j the two` threshold amplifier sections coupled to cathodes 1:23. and 126 are biased to be normally cut off except when the input signal` from the discriminator exceeds apredetermined amplitude. The eut-off level for the amplifier is: indicated by: dash line 163 in Figure 3. A somewhat higher amplitude, indicated in Figure 3 by line 164-,v is required` to. operate relays 37-41! (Figures 1 and2), since aminimum current is required to actuate the relays. The requisite. negative bias inthe illustrated embodiment is providedby the connection of the common terminal ofi resistors 131 and 132 to the negative source C'- of control power supply 51.

Under well-controlled environmentali conditions it is. only necessary to distinguish between'thecomponents of the amplitude-limited signal from. limiter 341011- the basis of frequency; in which case the;V threshold amplifier or amplitude-discriminator device comprising double triode may be connected directly to resistors 131 and 132 without providing the intervening integrating network shown in the preferred embodiment. The. control' system may then be triggered, however', by extraneous ultra-sonic signals having a frequency approximately equal tothe selected acoustic operating frequencies or' byV noise: at approximately 114 kc. or 117 kc. in the output from limiter 3d. In a more normal environment; the system might thus be triggered intospuriousoperation byI acoustic signals of very short duration producedV by the jingling of coins or keys or from other sources'. The system might also be falsely actuated by intermittent signals withinthe operating acoustic frequency ranges. In order to` avoid this possibility of malfunction of the system and take advantage of the slow decay of the ultra-sonic-signalproduced by the transmitter, thek integrating network. is utilized to average the output signall from the frequency discriminator comprising tube'. 122 over a' predetermined period of time, preferably somewhat shorter than the time constant of the acoustic transmitter. By properly selecting the circuit parameters for the integrating network and the threshold or firingv levels for the two sections of amplifier tube. 1411,. segregation network. 35 may be made responsive only to signals of predetermined minimum duration and duty cycle.

The efect of the integrating circuit is illustrated in Figure 4, in which solid-line curve represents the output voltage of tlre'frequency discriminator a function of time for un ampl-itudelimited signal at 114 kc. correspending to a received actuating signal) of 38 kc. from the ystem transmitter. Dash-line curve 166, on the other hand, indicates a typical noise signa-l which might occur over a similar time interval; the-noise signal may'vary substantially in frequency and/ or amplitude during that p'cri.- od and, as a consequence, the output voltage from the frequency Adiscriminator iluctuates considerably. The firing level for the threshold amplier comprising tube M) is indicated by dash-line 164; as indicated by the shaded areas 167 and 16S, the noise signall would actuate the amplitier section comprising electrodes 141', 143 and 145 (Figure 2) even in the absence of a desired actuating sig,- nal from the transmitter, if no integrating network were interposed between tube 1d() and the output resistors 13.1 and 132i of the frequency discriminator.

The integrated signal impressed upon control elect-rode 1.43 of tube 14u in response to input signal 165- is illus trated by dash line 169. As indicated, control electrode 143 is driven to a potential substantially above cut-off level 164 for a period of time substantially equal to the period` of conduction which would be provided by signal 165 if no integrating network were present, althoughthe period of conduction for the amplifier is somewhat delayed. r[he periods of spurious actuation indicated byshaded areas 167 and 163-, however, are completely eliminated by the integrating circuit,v as shown bycurve 1:76; which represents the integrated Voltage applied to control electrode 143 as a' result ofthe noise-induced output voltage 166 from the frequency discriminator. lt is thus apf parent that the integration` circuit. ofy segregation-network 35 renders the system responsive only' to signals of predetermined minimum duration as indicate-d by the time period 171 in Figure 4.

At the same time, the output signal' from the frequency discrimination. portieri of circuit 35 must have a minimum duty cycle in order to trigger the system. For a circuit having the characteristics illustrated in Figure 4, the minimum duty cycle for rapidly fluctuating signals from the discriminator is determined by the relative amplitudes of maximum limiter output level 165 and. relayy actuation level 164 andis: equal to approximately 65%. Thatis, a fluctuating signalY of maximum. amplitudemust be present 1l 65% of the time during a period atleast equal to the time .constant `of the integrating circuit, generally indicated at 172. It has been found that a minimum duty cycle of v50%, determined by establishing relay actuation level 164 :at at least 50% of limiter 165, should be maintained to provide adequate noise immunity.

It is thus apparent that the first section of tube 140 cornprising cathode 141, control electrode 143 and anode 145 is rendered conductive only when the amplitude-limited signal applied to circuit 115 from limiter 34 is sufciently close to the frequency corresponding to the selected acoustic operating frequency (38 kc.) and has a predetermined minimum duration and duty cycle. Consequently, a control potential or current is applied to operating -coil 147 of relay 37 only when these conditions obtain, so that muting relay 37 is actuated between its different operating conditions only in response to the desired acoustic signal from transmitter 30 (Figure l). Similarly, the vother section of amplier tube 140 comprising electrodes 142, 144,

and 148 (Figure 2) is rendered conductive only in rels'ponse to a received acoustic signal meeting the same requirements as to duration and duty cycle and having a frequency very nearly equal to 39 kc., so that coil 149 of ott-off relay 38 is actuated only at the desired times.

egregation network 36 functions in exactly the same manner to actuate the two m-otor-control relays 39 and 4,0 only in response to signals of predetermined duration and duty cycle having frequencies approximately equal to 40 and 41 kc. respectively. In this connection, it should again be noted that the two segregation circuits 35 and 36 may be identical in construction except that the resonant frequency for circuits 116 and 150 is made equal to 121.5 kc. instead of 115.5 kc.

One problem presented in the circuit illustrated in Figure 2 is that of imbalance between the output signals developed by the two frequency discriminator circuits., which is generally attributable to the plate-to-ground capacitance of limiter tube 102. T-his la-ck of balance between the two frequency discriminators can be substantial and can present a severe problem in the control system receiver. lt has been discovered, as described and claimed in the copending application of Robert Adler and John G. Spracklen, Serial No. 632,124, tiled January 2, 1957, for Frequency Discriminating System, and assigned t-o the same assignee as the present application, that the discriminators of networks 35 and 36 can be constructed to effectively neutralize this capacitive effect by suitable positioning of the inductance coils of the frequency discriminator tuned circuits. In essence, this is accomplished by locating the coils of resonant circuits 1.15 and 116 relatively close to each 4other and with connections of proper polarity so that the mutual inductance linking the two frequency discriminators eectively compensates for the plate capacitance of the limiter tube. in one satisfactory construction employed for this purpose, the four coils of the discriminator circuits are all aligned in a single row, the two inside coils constituting the inductances of circuits 11.5 and 116. With this arrangement, the coil spacing may be adjusted to provide the necessary controlled coupling and, if there is insuicient space available in the control receiver chassis to avoid overcoupling, excess mutual coupling can ne compensated oy adding a relatively small capacitor be tween limiter tube anode 112 and ground.

Of course, the remote control system is only as good as its transmitter, and it has been found that complexity and delicacy of construction are highly undesirable in this portion of the system. A preferred form of transmitter 200 for use in the remote control system of the invention is illustrated in Figure 5, lin which complete details are given for only one section 205 of the transmitting apparatus inasmuch as the four sections required to actuate the systems illustrated in Figures l and 2 may be essentially identical except for their resonant frequencies.

Acoustic transmitter section 205 comprises a vibrator element or rod 201 of homogeneous material having an overall 'length L equal to one-half wavelength of'sound in that material at a predetermined acoustic operating frequency. `Stated differently, the resonant frequency for vibrator rod'201 is approximately equal to the velocity of sound in the rod material divided by twice the length L. The diameter D of vibrator rod 201 is preferably made approximately equal to one wavelength at the resonant frequency in air, to provide for good efficiency in radiating the acoustic signal. Rod 201 is supported by a spring clamp 202 affixed to a bracket 203; spring clamp 202 should be located at the midpoint of the rod to permit 4longitudinal-mode vibration of the rod. This support structure including spring clamp 202 is described and claimed in the -copending application of Ole Wold, Serial No. 645,310, led March l1, 1957, for Ultrasonic Generator, and assigned to the present assignee. Bracket 203 is suitably aflixed t-o a support base 204.

Vibrator element 201 constitutes the resonant transmitting element for the rst transmitter section 205; the cornplete transmitter 200 includes three other essentially similar transmitter `sections which differ from section 205 only in the length of their individual vibrator elements. Provision is thus made for the four different acoustic operating frequencies necessary for actuating the control systems illustrated in Figures 1 and 2.

Transmitter section 205 further includes a pushbutton 206 axed to an operating rod 207 which is slidably supported in a pair of spaced brackets 208 and 209. A bias spring 210 is mounted on operating rod 207 to urge the pushbutton operating rod toward its normal or inactive position. A cam lever 211 is pivotally mounted on one side of operating rod 207 by means of a pin 212 and is biased for rotation in a counter-clockwise direction by a i spring 213 connected between a fixed point on operating rod 207 and an extension 214 of the cam lever. A stop element 215 mounted on lever 211 engages operating rod 207 to maintain the cam lever in its normal inoperative position as illustrated. An extension 216 of cam lever 211 engages a slot 217 in a strip rod 21S which is slidably mounted in brackets 208 and 209 for movement parallel to operating rod 207; trip rod 218 is biased toward its normal inoperative position, as illustrated, by a spring 219. One end of rod 218 extends beyond bracket 209 and is employed to support a striking element in the form of a hammer 220, hammer 220 being mounted on a resilient support bracket 221 affixed tothe trip rod. A damping element 225 is axed to rod 207 and extends into contact with vibrator element 201 at the end of the vibrator element adjacent hammer 220; the damping element may, forV example, comprise a length of resilient steel Wire. Certain details of this striking mechanism are described and claimed in the co-pending application of Robert C. Ehlers and Clarence W. Wandrey, Serial No. 645,091, tiled March 11, 1957, for Ultrasonic Transi mitter, and assigned to the present assignee.

To actuate transmitter section 205, pushbutton 206 is depressed in the direction indicated by arrow A. As the operating shaft 207 moves in this direction, cam lever 211 moves with it and the cam lever extension 216, by its engagement in slot 217, forces trip rod 218 in the same direction. When operating rod 207 has moved through a predetermined distance, an extension 222 on cam lever 211 engages a stop element 223 affixed to bracket 209. Consequently, continued movement of operating rod 207 in the direction indicated by arrow A causes the cam lever to rotate in a clockwise direction and releases cam extension 216 from slot 217 in trip rod 218. The trip rod vmoves rapidly in the direction indicated by arrow T, thereby impinging hammer 220 upon the end of vibrator rod 201 and exciting the vibrator rod into longitudinal mode vibration.`

Amplitude of vibration in rod 201 decays exponentionally in essentially the same manner as an electrical signal in a -shock-excited resonant circuit. The time constant for the resonator rod is dependent upon the rate at which` the vibratory energy stored ini therod is radiated intothe air andl upon'the internal damping. of the rod; These two factors, in turn, are functions of the'material-from which the'- vibrator rod is constructed. An extremely wide variety of materials may be employed' for this purpose, including metals, glass, ceramics, and others. In order to achieve a signal of useful amplitude over a'period of substantial duration, it is` desirable to select a material which exhibits relatively low internal damping at the desired opL erating frequency. A typical example of such material is aluminum, for whichthe time constant at a frequency of 40' kc. is of the order of 0.3 second and which exhibits a Q of the order of 35,000; The length of the Vibrator r'od, if made of aluminum, is relatively small at the suggested: operating frequencies in the 40 kc. range, being only approximately 2.5 inches.

The amplitude of the signal from transmitter section 265, as itimpinges up'on the microphone 62 (Figure 2) is inverselyv proportional to the distance separating the transmitter from the receiver portion of the control system. Obstructions in the direct line of sight, standing wave patterns caused by reflections and other factors may contribute'additional attenuation'. Consequently, a transmitter designed to provide an acoustic signal of sufficient amplitude to actuate the receiver reliably from a distance of 30 feet, for example, may continue to develop an output signal of sufficient strength to actuate the control system over a much longer time interval than desired when operated at a distance of only l feet from the receiver. In the case of the ratchet or step-type relays employed for the on-off and muting functions of the system, this effective extension of duration of the acoustic signal may have little or no adverse eect. With respect to the instantaneous-contact tuning control relays, however, it may easily cause the receiver to overshoot the desired tuning condition', thus Seriously impairing the effectiveness of the control system. Consequently, it is desirable to'include some means for damping the vibrator element of the transmitter under control of the transmitter operator. This damping eifect is readily achieved by means of damping element 225. When pushbutton 2de is depressed to actuate transmitter section 205, the damper element is removed from contact with vibrator rod 201 and consequently does not interfere with vibration of the resonant rod. As soon as pushbutton 206 is released, however, and returns to its normal position, damping element 225 again contacts vibrator rod 261 and effectively damps the vibrator so that it cannot continue to radiate an acoustic signal. Overshooting in the tuning controls is thus effectively avoided;

The longitudinal-mode vibratory system of the transmitter of Figure 5 provides marked advantages as compared to other types of acoustic signal generators. For example, prismatic bars subjected to vibration in the fundamental exure mode could be employed. For a device of this type, however, it is necessary to utilize a vibrator constructed from material much heavier than aluminum, since the rate of energy loss to the surrounding air is much higher for a flexure mode. Thus there would be no saving in Weight, even though the fleXure-mode device could be made substantially smaller in size. High-carbon steel could perhaps provide satisfactory performance, but it would introduce a substantial disadvantage due to the fact that the resonant frequency of a steel bar would be a function of the drawing temperature employed in fabricating the bar. ln addition, the frequency of a prismatic flexure bar is linearly proportional to the thickness of the oar and inversely proportional to the square of its length. Consequently, to obtain a frequency tolerance of one part in one thousand, a bar of approximately one inch length and one-half inch thickness would have to be machined to approximately i.00025 inch tolerances in both length and thickness. A longitudinal-mode vibrator such as rod 26E, on the other hand, has a resonant frequency which depends only on its length, being in- Cil vcrsely;l proportional thereto; Consequently, theresonant 'requency of,la 2L5 inch rod machined to 2.0025` inch, a ten times wider tolerance than the one mentioned above', is accurateito within one part in one thousand. ltwould be much too expensive to maintain the tolerances indicated for the exure mode resonator, so that individual adjustment of the frequency ofv each resonator would be necessary; this is not usually necessary with longitudinalmode vibrators. Considerations similar to those just given in connection with prismatic bars apply to other shapes such as bells, chimes and the like which utilize exure modes.

Figure 6 illustrates the segregation network sections of another embodiment of the invention which is otherwise `similar to that illustrated in Figure 2. l'n the embodiment of Figure 6, anode lf2 of tube 162 in limiter 34 is connected to the E+ supply through four series-connected coils 25d, 251, 252, and 253. Coil 2550 forms the primary winding of a transformer 254 having a secondary winding 25S which, with a capacitor 256, forms a parallel resonant circuit tuned to a frequency of 114 kcs. One terminal of coil 255 is connected to the anode 257 of a diode 258', the cathode 259 of the diode being returned tothe other terminal of coil 255 through a D. C, load resistor 26@ which is shunted by a capacitor 263. rl`he bottom terminal of coil 255 is also returned to the negative D. C. voltage supply C-. Cathode 259 of diode 258l is also connected to the control electrode 262 of an amplitude discriminator or threshold amplifier tube 263 through an integrating circuit comprising two resistors 26dl and 265, connected in series between cathode 259 and control electrode 262, and a pair of shunt capacitors 266 and 267. The cathode 263 of tube 263 is grounded and the anode 269 is connected to muting relay coil 147 (see Figure 2). Tuned-secondary transformer 2534, diode 25S, tube 263, and thenassociated circuitry comprise a frequency and amplitude segregation network indicated by dash outline 270.

Coil 251 constitutes the primary winding of a tunedsecondary transformer 273i which is coupled to a threshold amplitier comprising a tube 272, by means of a diode 273 and an integrating circuit, thereby forming a segregation network 27d which is essentially similar to circuit 27d. The embodiment of Figure 6 also includes a third similar amplitude and frequency segregation circuit 276 including coil 252 as the primary winding of a tuned-secondary transformer 277, a diode 27S, and a suitable integrating ci'rcuit coupling diode 278 to a threshold amplifier tube 279. A fourth segregation network 280 is included in the apparatus of Figure 6; discriminator 280 is essentially similar to devices 27th, 274, and 276. Network 230 includes a tuned-secondary transformer 28, a diode 252, and an integrating circuit connecting diode 282 to the control electrode of a threshold amplier tube 283.

ln many respects, operation of segregation circuits 270, 274, 276 and 230 is essentially similar to that of circuits 3S' and 36 of Figure 2. The secondary windings of transformers 25d, 271, 277, and 282i are tuned to ll4, 117, 120, and 123 kc. respectively to provide for selection between the four different third-harmonic signals supplied from limiter 34. ln each instance, the signal is rectified by the diode in the segregation circuit, integrated, and applied to the threshold amplier tube. Tubes 263, 272, 279 and 283 are each normally biased beyond cut-off by means of the connection of their control electrodes to negative bias source C- and are rendered conductive only when a signal within a limited frequency range is applied to the tuned transformer secondary, rectified, integrated, and supplied to the control electrode. Like the embodiment of Figure 2, therefore, the apparatus of Figure 5 applies a control signal to the individual relay coils only when the receiver is triggered by an acoustic signal within a limited frequency range and having a predetermined minimum duration and duty cycle. The

circuit of Figure 6 is somewhat less ynoise-immune than that of Figure 2, since any signal within the overall operating 'range of the segregation circuits 270, 274, 276, and 280 tends to produce a control signal of the correct polarity to render the threshold amplifier tubes conductive, whereas in the frequency discriminator circuits of Figure 2 noise having a relatively uniform frequency distribution produces no control potential at the threshold or amplitude-discrimination tubes, thereby affording additional protection against spurious operation. ln addition, because the frequency discriminator devices of Figure 2 each utilize two tuned circuits, they are, generally speaking, somewhat more frequency selective.

Of course, other types of acoustic transmitter apparatus can be employed in conjunction with the receiver control. systems of Figures l, 2, and 6 and the transmitter of Figure 5 may be employed in otheracoustic systems, but the two portions of the control system in combination complement each other to form a highly efficient and relatively inexpensive control system. The number of transmitter sections 205 required is of course dependent upon the number of control functions provided for in the receiver system. In this connection, it should be noted that additional segregation devices such as networks 35 and 36 (Figures l and 2) or circuits 270, 274, 276 and Ztl@ (Figure 6) may be added to the control system to permit regulation of other electrical circuits and, similarly, additional transmitter sections corresponding to section 2535 (Figure 5) may be added to the transmitter unit to actuate the additional segregation devices. By the same token, if it is desired to control fewer electrical circuits, the remote control system may be correspondingly simplified as by eliminating segregation network 36 (Figures l and 2) or devices 276 and 280 (Figure 6) in a system in which only two electrical circuits are to be controlled. A corresponding reduction in transmitter sections is of course made possible in the simpler control system. The control system is thus extremely flexible with respect to the number of electrical circuits it may be utilized to actuate.

The control system of the invention permits accurate and effective regulation of any number of electrical circuits without requiring substantial modification of the device in which those circuits are incorporated. The system is essentially immune to false actuation under normal operating circumstances and is quite simple and inexpensive in construction both at the transmitter and receiver terminals ofthe system.

Certain aspects of the invention disclosed herein are claimed in the copending application filed October 30, 1957, by the same inventor, for Remote Control System, and assigned to the assignee of the present application.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from the invention in its broader aspects. The airn of the appended claims, therefore, is to cover all such changes and modifications as fall within the true lspirit and scope of the invention.

I claim:

l. A control system, adapted for remote actuation by Van ultra-sonic signal of predetermined minimum amplitude and duration within a predetermined restricted frequency range, for controlling an electrical circuit actuatable between at least two different operating conditions, said control system comprising: means, comprising an input circuit including a microphone and a limiter coupled to said input circuit, for generating an amplitude-limited electrical signal having a predetermined frequency equal to an integral multiple, greater than one, of the -frequency of said ultra-sonic signal; means coupled to said limiter for developing a control signal only in response to a signal of predetermined minimum duration and duty cycle within a restricted frequency band including the frequency of said ,amplitude-limited signal, thereby distinguishing l5 said amplitude-limited signal from extraneous signals, both within and without said frequency band, which may appear in the output of said limiter; and means coupled to said last-mentioned means for utilizing said Icontrol signal to actuate said electrical circuit between its different operating conditions.

2. A control system adapted for remote .actuation by an ultra-sonic signal of predetermined minimum amplitude and duration within a predetermined restricted frequency range, for controlling an electrical circuit actuatable between at least two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating an electrical input signal having a frequency representative of said ultrasonic signal; means including a limiter, coupled to said input circuit, for developing an amplitude-limited signal having a predetermined frequency equal to an integral multiple, greater than one, of said input signal frequency; means coupled to said limiter for developing a control signal only in response to a signal of predetermined minimum duration and duty cycle within a restricted frequency band including the frequency of said amplitudelimited signal, thereby distinguishing said amplitude-limited signal from extraneous signals, both within and without said frequency band, which may appear in the output of said limiter; and means coupled to said last-mentioned means for utilizing said control signal to actuate said electrical circuit between its different operating conditions.

3. A control system, adapted for remote actuation by an ultra-sonic signal of predetermined minimum amplitude and duration within a predetermined restricted frequency range, for controlling an electrical circuit actuatable between at least two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating an electrical input signal having a frequency representative of said ultra-sonic signal; means including a limiter, coupled to said input circuit, for developing an amplitude-limited signal having a predetermined frequency equal to an integral multiple, greater than one, of said input signal frequency; means, including a frequency-discrimination device coupled to said limiter, an amplitude-discrimination device, and an integrating network coupling said frequency-discrimination device tosaid amplitude-discrimination device, for developing a control signal only in response to a signal of predetermined minimum duration and duty cycle within a restricted frequency band including the frequency of said amplitude-limited signal, thereby distinguishing said amplitude-limited signal from extraneous signals, both within and without said frequency band, which may appear in the output of said limiter; and means coupled to said amplitude-discrimination device for utilizing said control signal to actuate said electrical circuit between its different operating conditions.

4. A control system, adapted for remote actuation by a plurality of ultra-sonic signals of predetermined minimum amplitude and duration within a corresponding plurality of different predetermined restricted frequency ranges, for controlling a corresponding plurality of electrical circuits each actuatable between at least two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating a corresponding plurality of electrical input signals individually having a frequency representative of one of said ultra-sonic signals; means, including a limiter coupled to said input circuit, for developing a corresponding plurality of amplitude-limited signals each having a frequency equal to an integral multiple of the frequency of one of said input signals; means coupled to said limiter for developing a corresponding plurality of control signals only in response to signals of predetermined minimum duration and duty cycle within a corresponding plurality of different frequency bands individually including the frequency of one of said amplitude-limited signals,

thereby distinguishing said a "plitude-'liinited signals `from mitraneousf-l signals, lilith',withinl and u'u'thout said" fre"-` q'u'ency bands; which` may .appear in the output of said limiter; and means couple'clto said last-mentioned means for respectively utilizing said control signals to' actuate said electrical circuits between their aforesaid operating conditions.

L AV- control system, adapted for remote actuation by a plurality` f ultra-sonic signals of predetermined minimum amplitude and duration within `a corresponding plurality of different restricted frequency ranges, for controlling a corresponding plurality of electrical circuits each actu'atablebetween at least two different operating conditions, said control system' comprising: an' input circuit, including a microphone, for generating a corresponding plurality of electrical input signals individually having a frequency representative of one of said ultra-sonic signals'; means including a limiter', coupled to said input circuit, for developing a corresponding' plurality of amplitude-limited signals each having a frequency equal to an integral multiple of the frequency of one of said input signals; means' comprising `a pair of series-coupled frequency discriminators coupled' to said iimiter for developing a corresponding plurality of control signals only in response to signals of predetermined minimum duration and' duty .cycle within a corresponding plurality of different frequency bands individually including the frequency of one of said amplitudeilimited signals, thereby distinguishing said amplitude-limited signalsv` from extraneous signals, both within and without said frequency bands, which may appear inthe output of said limiter; and means coupled to said last-mentioned means for respectively utilizing said control signals to actuate said electrical circuits between their aforesaid operating conditions.

6. A control system, adapted for remote actuation by an ultra-sonic signal of predetermined minimum amplitude and duration within a predetermined restricted frequency range, for controlling an electrical circuit actuatable between at least two diiferent operating conditions, said control system comprising: an input circuit, including a microphone, for generating an electrical input signal having a frequency representative of said ultra-sonic signal; a limiter, coupled to said input circuit, comprising a harmonic generator having an output current vs. input voltage characteristic comprising two control voltage ranges of substantially zero transconductance separated by a control voltage range of high transconductance, for developing an amplitude-limited signal having a frequency equal to an odd integral multiple of said input signal frequency; means coupled to said limiter for developing a control signal only in response to a signal of predetermined minimum duration and duty cycle within a restricted frequency band including the frequency of said amplitude-limited signal, thereby distinguishing said arnplitude-limited signal from extraneous signals, both within and without said frequency band, which may appear in the output of said limiter; and means coupled to said last-mentioned means for utilizing said control signal to actuate said circuit between its different operating conditions.

7. A control system, adapted for remote actuation by two ultra-sonic signals of predetermined minimum amplitude and duration and falling within two distinct restricted frequency ranges, for individually controlling two electrical circuits each actuatable between at least two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating electrical input signals having respective frequencies representative of said ultra-sonic signals; means including a limiter, coupled to said input circuit, for developing two amplitude-limited sigals individually having a frequency equal to an integral multiple of one of said input signal frequencies; means, including a balanced frequency discriminator having two distinct frequencyresponse peaks respectively corresponding to said fre quencies of said amplitude-limited signals, coupled to'said lir'nitei for developing two individual cont'rol'signals only in response4 to signals of predetermined minimum duration' and duty cycle within a pair of different restricted frequency bands individually including the frequency of one of saidl amplitude-limited signals, thereby distinguishing-` said two amplitude-limited signals from extraneous signals, both within and without said frequency bands,- which may appear in the output of said limiter; and means coupled to said last-mentioned means for utilizing said two control signals to actuate said electrical circuits, respectively, between their aforesaid operating conditioiis'.

8`. control system, adapted for remote actuation by an utra-sonic signal of predetermined minimum amplitude and duration within a predetermined restricted frequency range, for controlling an electrical circuit actuatable between at leas't'two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating an electrical input signal having a" frequency representative of said ultra-sonic signal;` means including av limiter, coupled to said input circuit, fof developing an amplitude-limited signal having a frequency equal to an integral multiple of said input signal frequency; means coupled to said limiter for distinguishing 4said amplitude-limited signal from extraneous signals', both within and without a restricted frequency bandqinc'luding the frequency of said' amplitude-limited signal, which may appear in the output of said limiter, said: means including'v a' frequency-discrimiriation' device coupled tov said limiter for developing an output signal in which'frequency'componetsoutside of said frequency band are substantially attenuated in relation to fifequency components within said band, an integrating network coupled to said frequency discrimination device for developing an integrated signal indicative of the average amplitude of said output signal from said frequencydiscrimination device, and an amplifier, coupled to said integrating network, for developing a control signal only when said integrated signal exceeds a predetermined threshold value corresponding to a 50% duty cycle for said amplitude-limited signal; and means coupled to said lastmentioned means for utilizing said control potential to actuate said electrical circuit between its different operating conditions.

9. A control system, adapted for remote actuation by a plurality of ultra-sonic signals of predetermined minimum amplitude and duration within a corresponding plurality of different predetermined restricted frequency ranges, for controlling a corresponding plurality of electrical circuits each actuatable between at least two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating a corresponding plurality of electrical input signals individuaily having a frequency representative of one of said ultra-sonic signals; means, including a limiter coupled to said input circuits, for developing a corresponding plurality of amplitude-limited signals each having a frequency equal to an integral multiple of the frequency of one of said input signals; means, including a frequency discriminator having an input circuit tuned to the median frequency between an adjacent pair of said plurality of amplitude-limited signals, coupled to said limiter for developing a corresponding pair of control signals in respouse only to signals of predetermined minimum duration and duty cycle within a corresponding pair of different frequency bands individually including the frequency of one of said pair of amplitude-limited signals, thereby distinguishing said amplitude limited signals from eXtraneous signals, both within and without said frequency bands, which may appear in the output of said limiter; and means coupled to said last mentioned means for respectively utilizing said control signals to actuate said elec- I9 trical circuits between their aforesaid operating conditions.

10. A control system, adapted for remote actuation by a plurality of ultrasonic signals of predetermined minimum amplitude and duration within a corresponding plurality of different predetermined restricted frequency ranges, for controlling the corresponding plurality of electrical circuits each actuatable between at least two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating a corresponding plurality of electrical input signals individually having a frequency representative of one of said ultrasonic signals; means, including a limiter coupled to said input circuits, for developing a corresponding plurality of amplitude-limited signals each having a frequency equal to an integral multiple of the frequency of one of said input signals; means, iucluding a frequency discriminator having an input circuit tuned to the median frequency between an adjacent pair of said plurality of amplitude-limited signals and responsive only to signals at frequencies within a pair of restricted frequency bands spaced respectively above and below said median frequency and respectively including said adjacent pair of signals, coupled to said limiter for developing a corresponding pair of control signals in response only to signals of predetermined minimum duration and duty cycle within said pair of frequency bands, thereby distinguishing said amplitude limited signals from extraneous signals, both Within and without said bands, which -may appear in the output of said limiter; and means coupled to said last mentioned means for respectively utilizing said control 4signals to actuate said electrical circuits between their aforesaid operating conditions.

11. A control system, adapted-for remote actuation by two ultra-sonic` signals of predetermined minimum amplitude and duration and falling within `two distinct restricted frequency ranges, for individually controlling two electrical circuits each actuatable between at least two different operating conditions, said control system comprising: an input circuit, including a microphone, for generating electrical input signals having respective frequencies representative of said ultra-sonic signals; means including a limiter, coupled to said input circuit, for developing two amplitude-limited signals individually having a frequency equal to an integral multiple of one of said input signal frequencies; means, including a balanced frequency discriminator having two distinct frequencyresponse peaks respectively corresponding to said frequencies of said amplitude-limited signals and responsive only to signals at frequencies corersponding to said frequency-response peaks, coupled to said limiter for developing two individual control signals only in response to signals of predetermined minimum duration and duty cycle within a pair of different restricted frequency bands individually including the frequency of one of said arnplitude-limited signals, thereby distinguishing said two amplitude-limited signals from extraneous signals, both within and without said frequency bands, which may appear in the output of said limiter; and means coupled to said last-mentioned means for utilizing said two control signals to actuate said electrical circuits, respectively, be-

tween their aforesaid operating conditions.

No references cited. 

